The invention relates generally to the dental alloys which are used for filling teeth from which decayed portions have been removed. More particularly, the invention relates to an improved dental alloy having both corrosion resistance and improved handling characteristics compared to alloys of the prior art.
The prior art emphasized the development of alloys which are corrosion resistant. While typical dental alloys are principally composed of silver and tin, they usually contain small amounts of copper and zinc. A typical alloy of the prior art would contain at least 65 wgt. % silver, about 1-2 wgt. % zinc, and about 2-4 wgt. % copper, with the remainder being tin. Such alloys are not completely resistant to corrosion. It has been found that increasing the copper content of such alloys provides increased strength and also avoids the formation of what is known in the art as the gamma-two phase, a tin and mercury phase which has low resistance to corrosion and thus may lead to early deterioration of fillings. Typical of such high copper alloys are those disclosed in U.S. Pat. No. 3,871,876 and U.S. Pat. No. 3,997,328. Such dental alloy compositions increase the copper content from the typical 2-4 wgt. % to the range of 8-27 wgt. % in the first-mentioned patent, and in the latter patent from 20-40 wgt. %.
While such alloys have improved corrosion resistance, another important characteristic of dental alloys is its handling characteristics. The success of a dentist in filling a dental cavity is related to the handling characteristics of the alloy after it is amalgamated with mercury. For example, the high copper alloy disclosed in U.S. Pat. No. 3,871,876 is typically produced by air atomization from the molten state which results in a spherical or spheriodal form for the finished alloy. It is characteristic of alloys having a spherical shape that they feel relatively soft to the dentist and appear to require delicate handling. They are sometimes difficult to pack into a dental cavity since they have a tendency to be forced up the wall of the cavity if too much pressure is exerted or an instrument is used which has too small a bearing area. Consequently, many dentists find that such spherical material is not well adapted to their individual technique. As a result, they may be unable to take advantage of the corrosion resistance inherent with spherical alloys having a high copper content.
One method of improving handling characteristics of conventional dental alloys is disclosed and claimed in U.S. Pat. No. 3,997,327. In that invention a major portion of spherical particles is combined with a minor portion of microcut irregular particles, or flakes. Typical dental alloys in the prior art generally have been of the flake type, which inherently requires a higher pressure in order to be packed into a dental cavity than is characteristic of the spherical particle type alloys. By combining spherical particles with flake particles having the same composition, it is possible to improve the handling characteristics of the resulting mixture. Such a combination, having a conventionally low copper content, has less resistance to corrosion than the higher copper content alloys previously discussed.
One object of the present invention is to provide improved handling characteristics to corrosion-resistant dental alloys.